Well, yes they can eat garlic but it is not recommended or preferred that they don't eat garlic. This also applies to onion, spring onion etc...

Us being a Phật Tử, should know this when cooking for the monks. There's no rules saying that they are not allowed to eat them but most or usually phật tử that does the cooking will not include them into the food.

I know the reason why but it's a bit tricky for me to explain, I'll just briefly tell you why. It's because garlic, onion, spring onion etc. contains this substance that stimulates your body in a specific way that should be avoided especially for monks.

But some phật tử, even though being an adult doesn't know this and will include garlic and the likes into Thay's food and Thay being compassionate will still eat it to keep the phật tử happy even though they know it's best not to eat it.

You can go and ask the people that do the cooking in temples and ask them, usually the old ladies. hehehehThey will tell you it's not good for the monks.

So in conclusion, us being a phật tử we should help the monks tử by making food with appropriate ingredients.

Hope that clears up your curiosity. If further explaining is needed ask.